The Superconducting Quantum Interference Device, or SQUID, is well known as a sensitive detector of weak magnetic fields. As indicated in FIG. 1, a SQUID is comprised of a superconducting loop containing one or more Josephson junctions (indicated by X in FIG. 1), and magnetic flux Φ is inductively coupled into the loop through a coupling inductor L. A Josephson junction is known to act as a lossless nonlinear inductance below its critical current Ic, and also exhibits nonlinear resistance above Ic. A one-junction SQUID (FIG. 1A) comprises a single junction and a loop, and exhibits a nonlinear impedance which depends on the flux Φ in a periodic manner, with periodicity Φ0=h/2e=2.07 fT-m2, where h is Planck's constant and e is the charge on the electron. The one-junction SQUID does not have a direct voltage readout since the junction is shunted by a lossless superconducting inductor, so it must be embedded in a radio-frequency (RF) circuit for its impedance to be measured. For this reason, this structure is sometimes called an RF-SQUID, although the flux Φ can be at any frequency down to DC. Another SQUID device is the two-junction SQUID (FIG. 1B), which is generally operated with a DC bias current greater than the device critical current Ic=Ic1+Ic2 of the two constituent junctions. This then exhibits a DC voltage output across the Josephson junctions, modulated by the flux Φ in a way that is again periodic in Φ0 (FIG. 1C). This two-junction SQUID was historically called the DC-SQUID, since it operates down to DC frequencies, although it may alternatively operate with flux modulation up to gigahertz radio frequencies. The DC SQUID is much more commonly used than the RF SQUID, so in general usage, the term SQUID commonly refers to two-junction SQUID. Such a SQUID may be used not only for low-frequency magnetic field detectors, but also for radio-frequency amplifiers and active radio antennas if appropriate inductive inputs are used.
Both high-Tc and low-Tc superconductor based SQUID-amplifiers have been studied during the past ten years [7], [8], [9], [15], [16], [17]. See also Hilbert, U.S. Pat. No. 4,585,999, “RF amplifier based on a DC SQUID”. However, the characteristics of the amplifiers are still far from desired performance values. Despite the fact that the noise temperature Tn≈1-3 K [15] is reasonably low, the dynamic range (amplitude ratio) D=(Tsat/Tn)1/2 of the amplifiers is strongly limited by their saturation temperature Tsat, which is as low as 100-150° K [7], [15], [16]. The other disadvantage of the SQUID-amplifiers is a narrow range of linearity of the transfer function. Implementation of a flux-locked-loop operating mode can substantially increase dynamic range and linearity, but at the same time the external feed-back loop will limit the maximum operation frequency to a few tens of megahertz at best. Therefore an internal negative feedback has been suggested in order to increase dynamic range and to linearize the transfer function of such an amplifier [8], [9]. However this is very problematic given typical low values of the SQUID-amplifier gain [7], [15], [16] of 10-15 dB, since higher amplification gain is needed to effectively achieve the negative feedback. What is needed is a way to use arrays of SQUIDs to achieve both greater dynamic range and greater linearity, without requiring such negative feedback. Linearity is particularly important for processing signals at radio frequencies, where a nonlinear transfer function can give rise to undesired harmonics and intermodulation products.
One approach to overcoming the drawbacks of SQUID-amplifiers is associated with multi-element Josephson structures and arrays, including Superconducting Quantum Interference Filters (SQIF). See Schopohl, U.S. Pat. No. 6,690,162, “Device for High Resolution Measurement of Magnetic Fields”; Oppenlander, U.S. Pat. No. 7,369,093, “Superconducting Quantum Antenna.” A SQIF is an array (parallel, series or parallel-series) of DC SQUIDs with an unconventional array structure [10-12]. The SQIF voltage response is characterized by a single sharp peak. Contrary to the usual SQUID, which shows unique properties due to the strict periodicity of its structure, the unique properties of the SQIF result from just the opposite, an unconventional non-periodic array structure. SQIFs are therefore a new development of an intelligent network of SQUIDs. SQIFs certainly offer an approach to achieving increased dynamic range, but this approach does not offer a clear way to achieve linearization of these fundamentally nonlinear devices.